The color of a person's skin is determined by the amount of melanin, carotene, hemoglobin or the like, and among them, the melanin is the most important component. Melanin is synthesized in a melanocyte that exists in a basal stratum of skin (epidermis) and transferred to tissues or keratinocytes to reveal the color of the skin. The melanin that exists in an epidermis, which is in the outer position of skin, blocks and absorbs the ultraviolet rays to protect the inner skin organs under the dermis as well as removes free radicals generated in tissues to protect proteins and genes of the tissue.
However, the melanin generated by the stimulus of the inner or outer stress is not easily removed and remains until removed by keratinization of the skin, because melanin is very stable. Therefore, the over-generated melanin is an important source of trouble to make freckles or liver spots. The process of synthesizing a melanin is a kind of polymerization oxidation reaction that uses tyrosine or DOPA as a substrate and uses tyrosinase as an enzyme. When the amount of free radicals increases or inflammation reaction appears in the skin, or when ultraviolet rays hurt the skin, the synthesis of melanin increases.
As materials to inhibit or prevent melanin, polyoxy phenol compounds such as phenol, catechol and resorcinol, kojic acid, tocopherol, ascorbic acid and hydroquinone or the like are known. However, these materials show the anti-melanin effects by inhibiting the activities of the enzymes, and therefore, also show stimuli or toxicity to the skin in addition to the merit of inhibiting the activities of the enzymes.
In particular, hydroxy phenol compounds have such problems as being easily converted to quinone by tyrosinase to cause a harmful toxicity to the tissues.
In addition, the compounds that have anti-oxidation activities have the problems that they are not stable in the cosmetic formulation and easily change colors or denaturalize odors.
Therefore, the present inventors have studied to solve the problems of conventional whitening cosmetics that show stimuli and toxicity to skin, and to enhance the stability of whitening components in the cosmetic formulation. As one solution to the problem, the present inventors have studied to find compounds that can inhibit the synthesis of melanin and show whitening effects without depending on the anti-oxidation activity.
For this, the present inventors focused on gallic acid esters that are prepared from a kind of trihydroxy phenyl compound, gallic acid, and have various and differently positioned hydroxy groups, and also studied whitening effect and toxicity influenced by the position and number of the hydroxy groups in the gallic acid esters. As a result, the present inventors found that gallic acid esters having a protected tri-hydroxy group show excellent whitening effect. In addition, it was proved that whitening effect was obtained only when the compounds connected by ester bonds are thymol or carvacrol.
Based on the above studies, the present inventors prepared ester compounds by reacting 3,4,5-trimethoxy phenyl acetic acid, 3,4,5-trimethoxy cinnamic acid or 3,4,5-trimethoxy hydrocinnamic acid with thymol or carvacrol, and examined their whitening effects. As a result, the present inventors finally found that these esters have excellent effects of inhibiting the melanin synthesis without showing anti-oxidation activities.
In addition, they also found that these esters do not show cell toxicity, denaturalization of colors or odors and instability that conventional whitening agents belonging to anti-oxidation agents show, and when applied to skin, they do not show stimuli, deterioration or denaturalization.